The field of the invention is that of primary motor-driven pump units for pressurised water nuclear reactors (PWR). More precisely, this invention relates to ices, also referred to as active surfaces, of the main mechanical packing seal of the primary motor-driven pump unit.
In nuclear reactors, the primary pump generates the circulation of water in the primary circuit of pressurised water nuclear reactors. A dynamic system for sealing the shaft provides the seal between the primary circuit and the atmosphere. This system for sealing the shaft is a system with controlled leakage. It comprises three seals arranged in series. Each seal comprises two active surfaces that provide the main seal. One of the active surfaces, called rotary active surface, is mounted in a rotating unit integral with the shaft, the other active surface, referred to as floating, is mounted in a unit that does not rotate but that is free to be displaced axially in order to follow the possible axial displacements of the shaft.
The seal no. 1 provides most of the drop in pressure between the primary circuit and the atmosphere. It is of the hydrostatic type, with a film of water of a thickness of about 10 μm. The particular geometry of the surfaces of the active surfaces that provide the main seal makes it possible, when stopped as well as when rotating, the automatic adjustment of their separation which depends only on the ΔP of the seal. The active surfaces were initially made of alumina, but they are increasingly made of silicon nitride, which is more resistant to friction.
The seal 1 operates with a controlled leakage rate, of about 600 l/h during operation, thanks to the specific profile machines on its active surfaces. It makes it possible to pass from a pressure of 155 bars to a pressure of about 2 bars.
However, in the seals no. 1 of prior art, it was observed that a substantial deposit of iron oxide clogs the active surfaces and modifies the slope of these surfaces, which leads to a modification in the leakage rate.
The document “Deposition of hematite particles on alumina seal faceplates of nuclear reactor coolant pumps-laboratory experiments and industrial feedback” by Gregory Lefvre, Ljiljana S. Zivkovic and Anne Jaubertie, Hem. Ind., 2012, explains that this phenomenon of clogging is due to a phenomenon in two steps:                The particles are transported from the solution to the active surfaces by hydrodynamic, electrophoretic and thermophoretic phenomena;        Then they adhere to the sealing surfaces via physical-chemical interactions. In prior art, these interactions are considered as being substantially due to the fact that the particles of hematite are positively charged, while the surfaces of the active surfaces are negatively charged.        
In order to overcome this problem, document U.S. Pat. No. 7,287,756 proposes to add a catalyst to the surface of the active surfaces. This catalyst is more preferably one of or a mixture of the following compounds: rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold. According to document U.S. Pat. No. 7,287,756, the iron is present in solution in the form of FeOOH (goethite) and Fe2+ ions. The goethite would be deposited on the surface of the packing seals. In parallel, the Fe2+ ions would be oxidised by the oxygen into Fe3+ ions which would precipitate and consolidate the deposit. The deposit would then develop into hematite (Fe2O3). The use of catalysts would make it possible to dissociate the hydrogen present, with as a consequence reducing the chemical potential, preventing the oxidation of the ions Fe2+ and reducing the Fe3+ ions present in Fe2+, therefore preventing the depositing of the oxide from taking place.
The applicant has identified another mechanism for forming the deposit of oxide on the surfaces of the active surfaces. We therefore propose a solution that takes this mechanism of forming into account.
The invention aims to overcome the disadvantages of prior art by proposing an effective solution for preventing the clogging of the active surfaces of the seal no. 1 of the system for sealing the shaft of the primary motor-driven pump unit of a nuclear reactor.
To do this, the invention proposes to structure the surface of the active surfaces in such a way as to create asperities on the surface of the active surfaces that are smaller than the particles which are likely to attach to the surface of the active surfaces in order to form the clogging. The structuring aims to limit the points of attaching in such a way as to prevent these particles from being fixed on the surface of the active surfaces.
More precisely, the invention proposes an active surface for a packing seal for a system for sealing the shaft of primary motor-driven pump units of nuclear reactors, intended to provide the seal between the primary circuit and the atmosphere, with the active surface having at least one surface structured by an array of asperities, with each asperity having lateral dimensions between 10 nm and 5 μm, with each asperity having a height between 10 nm and 5 μm, with the distance between two consecutive asperities being between 10 nm and 5 μm.
Structuring the surface of the active surface in such a way as to have asperities that have such dimensions makes it possible to have on the surface of the active surface asperities that are smaller than the particles of Fe2O3, and therefore to reduce the points of attaching of these particles. Indeed, contrary to the phenomenon of attaching described in the documents of prior art, our experiments have shown that the particles of hematite do not directly attach to the surface of the active surfaces, but they attach to the Fe2+ ions which are themselves absorbed at the surface of the active surfaces. Indeed, the Fe2+ ions are attracted by the negative surface and donor of electrons of the active surfaces. The Fe2+ is a Lewis acid, it reacts with the oxygen groups present on the surface of the active surfaces and can in turn react with the colloidal or particulate Fe2O3 which has a strong electron donor component. The Fe2+ ions can then be absorbed on the surface of the particles of hematite and the continuous chain reaction, which causes the clogging of the active surfaces. Consequently, in order to prevent this clogging, the applicant proposes to prevent the attaching of the particles of Fe2O3 to the Fe2+ on the surface of the active surfaces.
The active surface according to the invention can also have one or several of the characteristics hereinafter taken individually or in any technically permissible combination.
The asperities can be holes or pillars.
When the asperities are holes, the height of the hole is called its depth. When the asperities are pillars, the form factor of the pillar, which corresponds to the ratio of its height over its lateral dimension must be more preferably less than 2 in order to prevent erosion phenomena.
The active surface according to the invention can be a rotary active surface or a floating active surface of the packing seal.
Advantageously, the entire surface of the active surface that is intended to be in contact with the film of water is structured.
The asperities can be nanometric asperities. In this case, the asperities have more preferably lateral dimensions between 10 nm and 1 μm, and a height between 10 nm to 1 μm. The distance between two consecutive asperities is preferably between 10 nm and 1 μm.
Indeed, the particles able to be attached to the Fe2+ on the surface of the active surfaces generally have dimensions between 50 nm and 5 μm, and it is preferable to have asperities that have dimensions between 10 and 50%, and more preferably of about from 20 to 30%, of the size of the particles of Fe2O3 in such a way as to prevent as much as possible the attaching of the particles of Fe2O3 on the surface of the active surface.
The asperities can also be micronic asperities. In this case, the asperities more preferably have lateral dimensions between 1 μm and 5 μm, and a height between 1 μm and 5 μm. The distance between two consecutive asperities is preferably between 1 μm and 5 μm.
The array of asperities is more preferably regular, i.e. it comprises a pattern that can be regularly reproduced, which facilitates the control of the array of asperities.
According to an embodiment, the asperities have more preferably the same dimensions and the distance between two consecutive asperities is preferably always the same, which facilitates the manufacture of the array of asperities.
According to a preferred embodiment, the surface of the active surface is micro- and nanostructured hierarchically by an array of micronic asperities of a lateral dimension and of a height between 500 nm and 5 μm, and preferably between 1 μm and 2 μm, with the distance between two micronic asperities being between 500 nm and 5 μm, and preferably between 1 μm and 2 μm. These micronic asperities are structured by nanometric asperities of lateral dimensions and of a height between 10 nm and 200 nm, and preferably between 50 nm and 100 nm. The distance between two adjacent nanometric asperities is between 10 nm and 200 nm. This double structuring at the nanometric and micrometric scale makes it possible to further reduce the attaching of the particles that can form the deposit.
The nanometric asperities can be holes or pillars.
The micronic asperities can be holes or pillars.
The active surfaces of the seal are more preferably made of silicon nitride.
Another aspect of the invention also relates to a method of protection against the clogging of the active surfaces of the packing seal for a system for sealing the shaft of primary motor-driven pumps of nuclear reactors, comprising a step of structuring the surface of the active surface in such a way as to carry out an array of asperities on the surface of the active surface. With each asperity having lateral dimensions between 10 nm and 5 μm, and a height between 10 nm and 5 μm, the distance between two consecutive asperities is between 10 nm and 5 μm.
The asperities can be holes or pillars.
When the asperities are pillars, they more preferable have a form factor less than 2 in order to prevent erosion phenomena.
According to different embodiments, this step of nanostructuring or microstructuring can be carried out according to:                A top-down approach by micro- or nanolithography, electron-beam lithography, X-ray lithography, deep ultraviolet lithography, nanoimprint lithography, interference lithography, via focused ion beam, laser scribing or via scanning probe microscopy. These steps can comprise a step of dry or wet etching.        A bottom-up approach (nanosphere or colloidal lithography) during which a mask of micro- or nano-objects (balls, nanoparticles, self-assembled block copolymers) can be used to replicate the asperities in the substrate via wet, dry or laser etching.        
The method can also comprise one or several of the following steps:                A step of reducing the sizes of the micro- or nano-objects        a step of depositing a layer serving as a mask on the micro- or nano-objects and on the non-covered substrate between the micro- or nano-objects        a step of removing micro- or nano-objects, leaving the mask on the substrate and forming an array of holes in the mask formed by the imprint of the micro- or nano-objects        a step of etching through the imprint of the nano-objects and the removing of the mask.        
The method can also comprise a step of depositing a protective layer against the clogging on the surface of the active surface. This protective layer is more preferably made of SiC, TiN, CrN, Ni or micro- or nanocrystalline diamond. In this case, it is more preferably the surface of the protective layer that is micro- or nanostructured.
For increased clarity, identical or similar elements are marked with identical reference signs over all of the figures.